Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, criu@openvz.org,
        bpf@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Jann Horn <jann@thejh.net>, Kees Cook <keescook@chromium.org>,
        =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= <berrange@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Matthew Wilcox <willy@debian.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Matthew Wilcox <matthew@wil.cx>,
        Trond Myklebust <trond.myklebust@fys.uio.no>,
        Chris Wright <chrisw@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Date:   Mon, 17 Aug 2020 17:04:20 -0500
Message-Id: <20200817220425.9389-12-ebiederm@xmission.com>
In-Reply-To: <87ft8l6ic3.fsf@x220.int.ebiederm.org>
References: <87ft8l6ic3.fsf@x220.int.ebiederm.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Subject: [PATCH 12/17] proc/fd: In fdinfo seq_show don't use get_files_struct
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When discussing[1] exec and posix file locks it was realized that none
of the callers of get_files_struct fundamentally needed to call
get_files_struct, and that by switching them to helper functions
instead it will both simplify their code and remove unnecessary
increments of files_struct.count.  Those unnecessary increments can
result in exec unnecessarily unsharing files_struct which breaking
posix locks, and it can result in fget_light having to fallback to
fget reducing system performance.

Instead hold task_lock for the duration that task->files needs to be
stable in seq_show.  The task_lock was already taken in
get_files_struct, and so skipping get_files_struct performs less work
overall, and avoids the problems with the files_struct reference
count.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/proc/fd.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index d9fee5390fd7..0b46eea154b7 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -28,9 +28,8 @@ static int seq_show(struct seq_file *m, void *v)
 	if (!task)
 		return -ENOENT;
 
-	files = get_files_struct(task);
-	put_task_struct(task);
-
+	task_lock(task);
+	files = task->files;
 	if (files) {
 		unsigned int fd = proc_fd(m->private);
 
@@ -47,8 +46,9 @@ static int seq_show(struct seq_file *m, void *v)
 			ret = 0;
 		}
 		spin_unlock(&files->file_lock);
-		put_files_struct(files);
 	}
+	task_unlock(task);
+	put_task_struct(task);
 
 	if (ret)
 		return ret;
@@ -57,6 +57,7 @@ static int seq_show(struct seq_file *m, void *v)
 		   (long long)file->f_pos, f_flags,
 		   real_mount(file->f_path.mnt)->mnt_id);
 
+	/* show_fd_locks() never deferences files so a stale value is safe */
 	show_fd_locks(m, file, files);
 	if (seq_has_overflowed(m))
 		goto out;
-- 
2.25.0